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 Post subject: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Mon 09, 2021 9:17 pm 
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Got a 1961 Bell Sounds Receiver in the shop today...Model 2425. Lots of output hum and no cathode voltage (unless they are running it to chassis thru 12AX7 heater...see next comment). I started looking at that circuit and looks like they are doing something funky there...running the four 6V6 cathodes to a electrolytic can (normal) and to the heater pin 5 of a 12AX7..??? That's a new one to me. I found a schematic of a 2420, but that is different than the 2425 which has 2 rectifier tubes---EZ80 and EZ81, and numerous other differences..I would like to see the 2425 schematic, from Sam's 540-8 if someone has it available. Thanks in advance!


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 Post subject: Re: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Tue 10, 2021 3:24 am 
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Bell 2425


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Bell 001.jpg
Bell 001.jpg [ 635.11 KiB | Viewed 493 times ]
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 Post subject: Re: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Tue 10, 2021 5:31 am 
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Location: NJ 08090
Interesting...Never saw this type of output tube cathode circuit driving the heaters of the two12AX7's (V-9/V10)---if I'm reading it right.????


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 Post subject: Re: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Tue 10, 2021 12:19 pm 
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I think if you can look at the power transformer side of the schematic somehow, you'll see a fliament winding with an arrow to "V" , which goes to the filaments in the 12AX7s. It's just drawn rather oddly, and it is a bit of an odd duck circuit as well. I don't recall seeing that configuration either.

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 Post subject: Re: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Tue 10, 2021 2:49 pm 
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A member here sent a copy of that Sam's folder....Found 2 bad 6V6's/replaced...and the odd cathode bias circuit is working now......Thanks!


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 Post subject: Re: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Wed 11, 2021 4:49 pm 
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Tbirdkid wrote:
Interesting...Never saw this type of output tube cathode circuit driving the heaters of the two12AX7's (V-9/V10)---if I'm reading it right.????


DC heaters for the phono preamp and no need for a big cathode resistor. 20V is less than the nominal but that results in lower noise.

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 Post subject: Re: Looking For Sam's 540-8 From 1961 For Bell 2425 Receiver
PostPosted: Aug Thu 12, 2021 2:32 pm 
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Location: NJ 08090
Quote:
DC heaters for the phono preamp and no need for a big cathode resistor.

OK...Now it makes sense...Clever design.


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